1. Field of The Invention
The present invention relates to a method of re-synchronizing a communication device comprised in a cellular communication system, more particularly to such re-synchronization when the communication device wakes up from a sleep mode after initial synchronization to a radio base station comprised in the cellular communication system.
2. Description of The Related Art
Cellular mobile radio or communication systems are well-known. Such cellular mobile radio systems comprise cells or radio zones, together covering a given geographical area. The cells include radio base stations that, through control and communication channels, establish and maintain communication links with mobile radio devices that are comprised in the mobile radio system, and communicate with the mobile radio devices through established communication links.
One type of a cellular communication system is a Universal Mobile Telecommunication System (UMTS) spread spectrum system proposed by the 3rd Generation Partnership Project (3GPP). In the 3GPP ETSI publication ETSI TS 125.213, V3.2.0 (2000-03), pp. 1-27 spreading and modulation is described for a 3GPP UMTS spread spectrum system. In such a system, but also in other systems, radio base stations communicate with mobile radio device using slots based transmission schemes. When the mobile radio device is powered on, it needs to synchronize its internal timing to the timing of the cellular communication system, more particularly to timing of a radio base station among radio base stations in the neighborhood of the mobile radio station that provides the best communication link. In order for a mobile radio station to be able to synchronize to and establish communication with a particular radio base station, in the proposed 3GPP system the radio base stations repetitively transmit synchronization burst on a so-called primary synchronization channel (PSCH) and a secondary synchronization channel (SSCH) in the form of so-called Golay sequences, and so-called Gold code scrambled signals on a primary common control channel (PCCCH), as described in the above 3GPP ETSI publication. All radio base stations transmit the same synchronization pattern at their PSCH, transmit different but not uniquely base station identifying synchronization patterns at their SSCH, and eventually base station identifying information at their PCCCH. In a spread spectrum system of a direct sequence type, such as in the proposed 3GPP system, the mobile radio device typically has a so-called Rake receiver with a number of Rake fingers to resolve multi-path received signals and to combine multi-path resolved signals so as to improve the signal-to-noise ratio of the received signal. The timing of the fingers in the Rake receiver is controlled by timing signals generated by a so-called spread spectrum searcher. The searcher is used for initial synchronization of the mobile radio device, upon powering up of the device. After initial synchronization, tracking mode synchronization is adopted. In between multiple slots, in the so-called idle mode, in order to save power, a mobile radio device enters a sleep mode by switching off a major part of its reception circuitry, including its local timing reference, usually a voltage controlled crystal oscillator, while keeping a master timer such as a counter running, but at a substantially lower clock frequency. In the sleep mode, the mobile radio device looses some of its synchronization. Therefore, before reception a next paging signal from the radio base station, the internal timing of the mobile radio device needs to be adjusted. More particularly, the master timer needs to be re-synchronized to the timing of the radio base station it was previously locked to and, possibly, was communicating with. In systems like the proposed 3GPP system, the re-synchronization process is the same as the initial synchronization process to synchronize to a particular neighboring radio base station, and is a three-step process that is performed by the searcher. Such three-step process is a time and power consuming process. Firstly, after the mobile radio device transitioned from idle mode into receive mode by switching on its timing reference and other circuitry that was switched off during idle mode, the spread spectrum searcher, e.g. implemented as a matched filter, searches for the primary synchronization channel. Within a time slot between successive transmission bursts from the radio base station, at a PSCH synchronization step, the matched filter resolves base station signals of neighboring base stations, without identifying a particular base station. Secondly, at a SSCH synchronization step, in which a matched filtering operation is followed by a fast Hadamard transformation, a non-uniquely base station identifying group code of each resolved base station is obtained. Finally, at a third, PCCCH information reception step, usually performed by a correlator correlating the received PCCCH information with different Gold scrambling codes, the best correlation match provides the desired radio base station. This three-step process is performed by the searcher both during initial synchronization and re-synchronization.
It is an object of the invention to provide a fast re-synchronization method, with a minimum number of steps and with minimum power consumption.
It is another object of the invention to provide, upon initial synchronization to a radio base station of the system, re-synchronization to the same radio base station when the communication device wakes up from a sleep mode entered after the initial synchronization.
It is still another object of the invention to provide re-synchronization through fitting of multi-path transmission patterns that are characteristic in time for a particular configuration of radio base stations.
In accordance with the invention, a method of re-synchronizing a communication device that is comprised in a cellular communication system is provided, said method comprising:
initially synchronizing a master timer of said communication device to a first timing reference of a radio base station comprised in said cellular communication system;
determining and storing a first channel profile of received multi-path, signals relative to said initial synchronization;
entering a sleep mode after said initial synchronization, in said sleep mode switching off a second timing reference that controls said master timer;
from said sleep mode, entering a receive mode, in said receive mode, switching on said second timing reference;
determining a second channel profile of received multi-path signals;
deriving a timing offset signal from a best fit obtained by fitting said first channel profile to said second channel profile; and
re-synchronizing said master timer on the basis of said derived timing offset signal.
The invention is based on the insight that multi-path transmission patterns that are characteristic in time for a particular configuration of radio base stations do not change over a relatively short period of time such as a period between transmission of multiple time slots, and thus can be used to derive a timing offset for re-synchronization of the communication device after it wakes up from a sleep mode.
Advantageously, the channel profiles are fitted using a mean-square error fitting method, more particularly a mean-square error method fitting replica of said second channel profile to said first channel profile, said replica being time shifted versions of said second channel profile as time shifted over a fraction of a reception time slot of said cellular communication system. Herewith, the timing offset signal is obtained by a simple and robust method.